The Redmonk methodology of looking at stack overflow and github seems to be far superior to the TIOBE search engine approach. There is a real story in the moves on this index: the rise of Swift and Kotlin, the stagnation of go and scala.
The Go stagnation is surprising. Is it because of its simpler syntax and hence less SO questions?
I'm seeing an opposite trend on the job market, Go is growing fast.
Your hypothesis seems confirmed by the fact Go ranking decreases on the StackOverflow axis, but not on the GitHub axis.
Seems clear to me that Scala's decline is exactly due to the rise of Kotlin. Whatever interest I once had in Scala has evaporated now that Kotlin has arrived.